The present invention relates to residential electric circuit breakers and particularly to a cost improved current switching member therefor.
Traditionally, the elongated, pivotal current switching arm of an automatic electric circuit breaker has been formed of copper since it is included in the breaker internal circuit, in that it provides the current path between a movable contact affixed to its lower end and a flexible, conductive braid having one end affixed to the arm at a point intermediate its ends. The other end of the braid as affixed in electrical connection with overcurrent responsive breaker tripping means, typically a bimetal and an electromagnet, through which current flows enroute to the breaker load terminal. Completing the breaker internal circuit, the line terminal of the circuit breaker is electrically connected with a stationary contact which is engaged by the movable contact when the pivotal arm is manually operated to its closed circuit position via a spring-powered, typically toggle-type operating mechanism.
Since residential electric circuit breakers are enclosed in a molded, insulative case, high electrical conductivity of the current carrying movable contact arm is an absolute necessity in order to minimize watts loss. The heat developed within the molded case due to excessive voltage drop across the breaker internal circuit has a deleterious affect on the breaker parts, as well as upsetting the calibrated thermal trip setting.
In addition to high electrical conductivity, high thermal conductivity for the movable contact arm is also an important consideration. Since a significant source of heat is the voltage drop across the engaged stationary and movable contacts, it is important from the standpoint of long service life that they be adequately heat sinked such that heat be efficiently drawn away. This is especially so when a circuit breaker is utilized in switching duty applications, since the contacts are repeatedly being subjected to arcing. Insofar as the movable contact is concerned, the only available, effective heat sink is the arm.
It would be manifestly desirable to remove the circuit breaker movable arm from the breaker internal circuit and thus permit its fabrication from a material less expensive than copper. Achieving this would also avoid the significant material cost involved in the current practice of silver or tin plating the copper arm to discourage oxidation and corrosion. An approach to this end is disclosed in U.S. Pat. No. 3,930,211, wherein the movable contact is brazed to the end of a steel pivotal arm, and the braid is welded in direct electrical connection with the back side of the movable contact. While this design removes the arm from the breaker internal circuit, it is not seen that adequate heat sinking of the movable contact is provided, particularly when the circuit breaker is subjected to switching duty applications.
It is accordingly an object of the present invention to provide a cost improved current switching member for electric circuit breakers.
A further object is to provide a current switching member of the above character which provides a high electrical conductivity segment of the circuit breaker internal circuit.
Another object is to provide a current switching member of the above character which provides effective heat sinking of the movable contact supported thereby.
Yet another object is to provide a current switching member of the above character which is convenient to assemble and both efficient and reliable in service.
Other objects of the present invention will in part be obvious and in part appear hereinafter.